1. Technical Field
This invention is related to disk drive systems, and particularly to those disk drive systems adapted to receive a removable floppy disk characterized by a magnetic media which is subject to eventual destruction through head contact wear.
2. Description of Related Art
Magnetic disks are the most popular type of memory for home personal computers. The data capacity of a magnetic disk is limited by the number of bytes that may be stored in a given track length. Hard disks, characterized by a stiff media, may be rotated at relatively high rotational rates, and therefore provide the advantages of a fast data rate (short data access time) and an aerodynamic cushion between the disk and the read/write head (which virtually eliminates head contact wear on the disk). However, hard disks suffer from the disadvantage that they are relatively costly and, until recently, were not removable from the disk drive. Some recent development efforts have produced removable hard disks, but these are relatively more expensive.
Floppy disks have, since their inception, been removable from the disk drive; and are relatively inexpensive and therefore much more attractive in the consumer personal/home computer market. However, floppy disks suffer from the disadvantage that--generally--they must be rotated by the disk drive at a relatively slower rotational rate. Therefore, there is little aerodynamic cushion between the disk and the head so that the disk wear due to head contact is relatively great. The magnetic read/write head rides on the magnetic media on the floppy disk surface and eventually, through attrition, abrasion and stress, wears it out, so that the disk becomes useless. The outermost disk tracks receive the most frequent wear because they typically store the disk file allocation tables so that the read/write head must spend a higher percentage of its time in the outermost tracks. The innermost tracks suffer from the disadvantage that the disk media is less compliant, so that the rate of head contact wear (i.e., the wear per unit time during head contact) is more rapid in the innermost disk tracks.
Various solutions of limited efficacy are known in the art. These solutions recognize that a personal computer needs to access data on the disk sporadically, so that there are a number of times during which the disk drive system is essentially idle, possibly for several seconds or minutes at a time. By minimizing or eliminating head contact with the disk during idle times when no such contact is required, overall disk wear may be reduced and the disk life extended.
An obvious solution is to simply stop disk rotation whenever the personal computer leaves the disk drive idle. Unfortunately, this solution introduces a severe limitation in the data access time of the disk drive, since each subsequent request for data is delayed by about 1 second during which the disk drive system must spin-up the disk and re-position the read/write head. Due to the great competition among disk drive manufacturers with regard to disk drive performance specifications, characterized by data access time differences of milliseconds (and less) between competing products, the addition of one second to the minimum data access time would render such a system not commercially viable in this market. Thus, this obvious solution is really no solution at all.
Another seeming solution is to simply lift the heads from the disk during idle time. However, this suffers from the same type of disadvantage because the time required to re-contact and re-position the head on the disk during each subsequent request for data by the personal computer detracts from system performance.
A more viable solution is described in U.S. Pat. No. 4,445,188 in which, during idle time, the disk is not de-spun and the heads are not removed from the disk (so that there are no great increases in data access time). Instead, head contact wear is distributed across all of the tracks of the disk during idle time by progressively moving the head from the outermost disk tracks to the innermost tracks, one track at a time, during every idle time. This solution has the advantage, of course, that there are no large additional data access delays for spinning-up the disk for each data request or bringing the head back into contact with the disk for each data request. However, a three-fold problem remains unsolved.